Limiting current using an integrated MosFET

You may often see protection stages like hot swap controllers, surge protectors, electronic circuit protectors and ideal diodes in a current limiter circuit. These components mostly use external MOSFETs as switches for switching the current flow on and off, but also for limiting the current, in which case the switch works like a linear regulator. Such a switch must, however, ensure that the MOSFET is always operated within its safe operating area (SOA). If this is not the case, the semiconductor and thus the circuit will be damaged.

Unfortunately, it’s not always easy to select a suitable MOSFET and operate it in just such a way that it never leaves the SOA. The operating temperature, the voltage, the current and especially the time are all factors that influence this. They all have to be right in order to ensure safe operation. Figure 1, below, shows an SOA diagram for a typical N-channel MOSFET. Operation of the MOSFET below the lines shown is permitted.


Figure 1. A typical SOA of a MOSFET

One example is the MAX17523 from Analog Devices. It has two MOSFETs that can limit current to a value between 150 mA and 1 A. If the current flow reaches the limit, it is either cut off and resumed after a certain waiting period, or the current flow is interrupted continuously until the next switch-on, or the current is limited through a reduction in voltage. The internal MOSFET is then operated in the ohmic region. This is then a type of linear regulator function. In each of these adjustable limitation modes, the internal MOSFET is always in its SOA and is not damaged. And no elaborate calculations or evaluations are required.


Figure 2. A simplified circuit diagram of a dedicated current limiter IC.

Limiting currents in a circuit is not a problem if suitable highly integrated ICs are used. It also makes sense to combine this type of circuit with a DC-to-DC converter if the converter doesn’t have an adjustable current limiter.